DIOXYGEN UPTAKE AND TRANSFER BY CO(III), RH(III) AND IR(III) CATECHOLATE COMPLEXES

A large number of five-coordinate metal catecholate complexes of the general formula [(triphos)M(Cat)]Y have been synthesized and characterized by chemical, spectroscopic and electrochemical techniques (M=Co, Rh, Ir; Cat=9,10-phenathrenecatecholate, 1,2-naphthalencatecholate, 3,5-di-tert-butylcatecholate, 4-methylcatecholate, 4-carboxycatecholate-ethylester, tetrachlorocatecholate; Y=BPh4, PF6; triphos=MeC(CH2PPh2)3). All of the compounds undergo electron-transfer reactions that encompass the M(III), M(II) and M(I) oxidation states of the metal, and the catecholate, semiquinone and quinone oxidation levels of the quinoid ligand. Paramagnetic Ir(III) semiquinonate complexes, [(triphos)Ir(SQ)]2+, and Ir(II) catecholates, [(triphos)Ir(Cat)], have been characterized by X-band ESR spectroscopy. The reactions of the metal catecholates in non-aqueous media with dioxygen have been investigated. With very few exceptions, all of the compounds react with O2 to give adducts of the general formula [(triphos)M(OO)(SQ)]Y. An X-ray analysis bas been carried out on [(triphos)Ir(OO)(PhenSQ)]BPh4 (Phen=9,10-phenanthrenesemiquinonate). In the complex cation, the metal is octahedrally coordinated by the three phosphorus atoms of triphos and by three oxygen atoms, one from O2 and the other two from the catecholate ligand that has attained a semiquinoid character. The electrochemical behavior of the dioxygen adducts has been studied in detail. Depending on the E-degrees' values relative to the M(III)(SQ)/M(III)(Cat) couples of the parent metal catecholates, the dioxygen adducts undergo either a one-electron oxidation to give o-quinone complexes [(triphos)M(Q)]3+ and superoxide ion (O2-) or a two-electron oxidation to give [(triphos)M(Q)]3+ and O2. Several factors have been found to affect the O2 uptake by metal catecholates. Of particular importance are: (i) the coordination number of the metal; (ii) the basicity of either the catecholate ligand or metal; (iii) the temperature; (iv) the pressure of dioxygen. The role of each factor has been analyzed and rationalized. The transport of dioxygen from one metal catecholate to another has been studied. A mechanistic interpretation for the formation of the [(triphos)M(OO)(SQ)]+ complexes is proposed in light of a large crop of experimental data and molecular orbital considerations.